UGANDA STANDARD
 
        US ISO 6637
First Edition
2009-mm-dd
 
Reference numbe r 
    US ISO 6637:1984
 © UNBS 2009
  
Fruits, vegetables and derived products — Determination of 
mercury content — Flameless atomic absorption method 
 US ISO 6637: 1984 
 
© UNBS  2009 – All rights reserved ii Compliance with this standard does not, of itself confer immunity from legal obligations 
A Uganda Standard does not purport to include all necessary provisions of a contract. Users are 
responsible for its correct application  
© UNBS 2009 
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     US ISO 6637:1984  
© UNBS  2009 - All rights rese rved                iii National foreword 
Uganda National Bureau of Standards (UNBS) is a parastatal under the Ministry  of Tourism, Trade and 
Industry established under Cap 327, of the Laws of U ganda. UNBS is mandated to co-ordinate the elaboration 
of standards and is  
(a) a member of International Organisation for Standardisation (ISO) and  
(b) a contact point for the WHO/FAO Codex A limentarius Commission on Food Standards, and  
(c) the National Enquiry Point on TBT/SPS Agr eements of the World Trade Organisation (WTO). 
The work of preparing Uganda Standards is carried out through Technical Committees. A Technical 
Committee is established to deliberate on standards in a gi ven field or area and consists of representatives of 
consumers, traders, academician s, manufacturers, governme nt and other stakeholders. 
Draft Uganda Standards adopted by the Technical Committ ee are widely circulated to stakeholders and the 
general public for comments. The committee reviews the comments before recommending the draft standards 
for approval and declaration as Uganda Standards by the National Standards Council. 
This Uganda Standard, US ISO 6637, Fruits, vegetables and derived products — Determination of mercury
content — Flamel ess atomic absorption method,  is identical with and has been reproduced from an
International Standard, ISO 6637:1984, Fruits, vegetables and derived products — Determination of 
mercury content — Flameless atomic absorption method, and adopted as a Uganda Standard.  
This standard was developed under the subcommittee on fruits and vegetables by the Food and agriculture 
Standards Technical Committee (UNBS/TC 2). 
Wherever the words, "International Standard" a ppear, they should be replaced by "Uganda Standard." 
 International Standard 
INTERNATIONAL ORGANIZATION FOR STANDARDIZATlONWVlE)I(LIYHAPOAHAR OPTAHW3ALIMR n0 CTA~AAPTbWWIM@3RGANlSATlON INTERNATIONALE DE NORMALISATION 
Fruits, vegetables and derived products - Determination 
of mercury content - Flameless atomic absorption 
method 
Fruits, legumes et pro&its d&iv& - D&termination de la teneur en mercure - Mhhode par absorption atomique sans flamme 
First edition - 1984-06-01 
UDC 634.1/635.6 : 543.422 : 546.49 Ref. No. IS0 6637-1984 (E) 
Descriptors : food products, fruit and vegetable products, tests, determination of content, mercury, atomic absorption spectroscopic method. 
Price based on 5 pages Foreword 
IS0 (the International Organization for Standardization) is a worldwide federation of 
national standards bodies (IS0 member bodies). The work of developing International 
Standards is carried out through IS0 technical committees. Every member body 
interested in a subject for which a technical committee has been authorized has the 
right to be represented on that committee. International organizations, governmental 
and non-governmental, in liaison with ISO, also take part in the work. 
Draft International Standards adopted by the technical committees are circulated to 
the member bodies for approval before their acceptance as International Standards by 
the IS0 Council. ‘\ *. 
International Standard IS0 6637 was developed by Technical Committee ISO/TC 34, 
Agricultural food products, and was circulated to the member bodies in August 1982. 
It has been approved by the member bodies of the following countries : 
Australia 
Austria 
Canada 
Czechoslovakia 
Egypt, Arab Rep. of 
Ethiopia 
France 
Germany, F. R. 
Hungary 
India Iran 
Ireland 
Israel 
Kenya 
Korea, Dem. P. Rep. of 
Korea, Rep. of 
Malaysia 
Mexico 
Netherlands 
New Zealand 
No member body expressed disapproval of the document. Philippines 
Poland 
Romania 
South Africa, Rep. of 
Spain 
Tanzania 
Turkey 
USA 
USSR 
Yugoslavia 
0 International Organization for Standardization, 1984 
Printed in Switzerland INTERNATIONAL STANDARD IS0 6637-1984 (E) 
Fruits, vegetables and derived products - Determination 
of mercury content - Flameless atomic absorption 
method 
1 Scope and field of application 
This International Standard specifies a method for the determi- 
nation of the mercury content of fruits, vegetables and derived 
products. 
2 Principle 
Decomposition of organic matter in a sulfuric-nitric acid 
medium. Reduction of mercury(ll) to metallic mercury by the 
action of tin(ll) chloride. Entrainment of the mercury vapour by 
a current of air and determination by flameless atomic 
absorption in an enclosed apparatus. 
3 Reagents 
All the reagents shall be of recognized analytical quality and 
shall, with the exception of the standard mercury chloride solu- 
tions (3.6 and 3.7), be free from mercury. The water used shall 
be distilled water free from mercury, or water of equivalent 
purity. 
3.1 Sulfuric acid (ezO = 1,84 g/ml). 
3.2 Nitric acid (ezO = I,38 g/ml). 
3.3 Nitric acid, 5 % (V/ V) solution. 
3.4 Tin(H) chloride, 100 g/l solution. 
3.5 Urea, 400 g/l solution. 
3.6 Mercury(H) chloride, standard solution corresponding 
to 1 g of mercury per litre. 
In a 500 ml one-mark volumetric flask, dissolve 0,676 8 g of 
mercury(H) chloride in the nitric acid solution (3.3) and dilute to 
the mark with the same nitric acid solution. 
1 ml of this standard solution contains 1 mg of mercury. 
3.7 MercuryW chloride, standard solution corresponding 
to 100 pg of mercury per litre. 
At the time of use, prepare a 1 /lo 000 ( V/ V) dilution of the 
standard mercury(ll) chloride solution (3.6) in the nitric acid 
solution (3.3). 
1 ml of this standard solution contains 0,l pg of mercury. 4 Apparatus 
The glassware used shall be washed beforehand with hot con- 
centrated nitric acid and rinsed with water. 
Usual laboratory equipment, and 
4.1 Mechanical grinder, the internal lining and blades of 
which are of polytetrafluoroethylene (PTFE). 
4.2 Decomposition apparatus (see figure 1). 
The apparatus is made of borosilicate glass and comprises four 
elements joined by conical ground joints, as follows: 
a) Element (A) is a modified Soxhlet extractor of capacity 
200 ml, fitted with a stop-cock and a side tube linking this 
element directly to the round-bottomed flask (D); 
b) Element (B) is a dropping funnel of capacity 75 ml, fit- 
ted to the second neck of the round-bottomed flask (D); 
c) Element (C) is a condenser 35 cm long, fitted to the top 
of the Soxhlet extractor (A); 
d) Element (D) is a round-bottomed flask, of capacity 
500 ml, fitted below the Soxhlet extractor (A), and in the 
second neck (30 mm from the first neck) of which is fitted 
the dropping funnel (B). 
When the stop-cock of the Soxhlet extractor (A) is open, the 
apparatus is in the reflux position; when it is closed, the 
Soxhlet extractor (A) enables the condensed water and acid 
vapours to be retained. 
4.3 Flameless atomic absorption apparatus (mercury 
analyser system). 
The apparatus comprises a spectrometric system and deter- 
mination flasks. The arrangement of the apparatus is shown in 
figure 2. 
4.3.1 The spectrometric system comprises a mercury vapour 
lamp, the beam of which passes through an absorption cell. 
The variation in energy transmitted through the cell is 
measured by a phototube sensitive to ultraviolet radiation. A 
filter placed in front of the phototube isolates radiation of 
wavelength 253,7 nm. The equipment also has a read-out 
device. 
The absorption cell shall have an internal diameter of 25 mm 
and a length of 115 mm and shall have silica windows. 
1 IS0 66374984 (El 
4.32 The determination flask is equipped with a bubbler and 
is connected to a closed circuit in which the metallic mercury is 
released and entrained by circulation of air thus ensuring, by 
recycling, uniform distribution of the mercury in the circulating 
air. The absorption cell (see 4.3.1) is interposed in this circuit. 
4.3.3 Any other atomic absorption 
required sensitivity may also be used. apparatus having the 
4.4 Pipettes and burettes, of appropriate capacities. 
4.5 One-mark volumetric flasks, of capacity 100 ml. 
4.6 Analytical balance. Allow the reaction to proceed very gently so as to avoid any 
loss of mercury through the transfer of particles supplied by the 
nitrous vapours in the condenser. Continue decomposition 
under reflux for about 30 min until the liquid has a uniform ap- 
pearance. If the mixture turns brown, add several drops of 
nitric acid (3.2) through the dropping funnel (B) until the colour 
is discharged. Allow to cool. 
5.3.1.2 Carefully add 10 ml of a mixture of equal parts of the 
nitric acid (3.2) and the sulfuric acid (3.1). Heat with a small 
flame and add nitric acid (3.2) drop by drop if the digest turns 
brown. Continue heating until fibrous matter has apparently 
been destroyed. Close the stop-cock of the Soxhlet extractor 
(A) to trap the water and acids and continue heating. The 
decomposition liquid will become more concentrated. If the li- 
quid turns brown, add several drops of nitric acid (3.2) in just 
sufficient quantity to discharge the colour. Continue heating 
until the nitrous vapours are eliminated and there is a concen- 
tration of white fumes above the decomposition liquid. 
5 Procedure 
NOTE - Waxes and fats cannot be completely destroyed by hot acids. 
5.1 Preparation of the test sample 
Mix the laboratory sample well. If necessary, first remove 
stones and hard seed-cavity walls, and pass the laboratory 
sample through the mechanical grinder (4.1). 
Allow frozen or deep-frozen products to thaw in a closed vessel 
and add the liquid formed during this process to the product 
before mixing. 
5.2 Test portion 
5.2.1 Liquid products 
Transfer, by means of a pipette (4.4), 10 ml of the test sample 
(5.1) to the round-bottomed flask (D) of the decomposition 
apparatus (4.2). 
NOTE - It is also possible to take the test portion 
weighing, to the nearest 0,Ol CL 10 g of the test sample. 
5.2.2 Doughy, solid or dehydrated products bY mass bY 
Transfer a mass of the test sample (5.1) corresponding to about 
5 g of product, weighed to the nearest 0,Ol g, to the round- 
bottomed flask (D) of the decomposition apparatus (4.2) and 
add 5 to 10 ml of water. 
5.3 Decomposition 
53.1 Decomposition of the test portion 
5.3.1.1 Place a few glass beads in the flask (D), and connect 
the flask to the rest of the decomposition apparatus (4.2). By 
means of the dropping funnel (B), add, drop by drop, 5 ml of 
the nitric acid (3.2). Start a fast flow of water through the con- 
denser (C) and turn the stop-cock of the Soxhlet extractor (A) 
to the reflux position; place a metal sheet with a hole of diame- 
ter approximately 5 cm below the flask, and heat with a small 
flame. 5.3.1.3 Control the heating so that the white fumes do not 
rise more than half way up the condenser (C). The liquid shall 
be colourless or pale yellow. Allow to cool. Carefully entrain the 
water and acids collected in the Soxhlet extractor (A) into the 
flask (D) by opening the stop-cock. Add 5 ml of the urea solu- 
tion (3.5) through the side neck, and boil under reflux for 
30 min. Allow to cool. 
5.3.1.4 Disconnect the apparatus and transfer the contents of 
the flask ID) into a volumetric flask (4.5). Ensure that no un- 
digested waxes or fats are transfered. Rinse the condenser (C) 
and the Soxhlet extractor (A) twice with 15 to 20 ml of the 
nitric acid solution (3.3), collecting the rinsings in the flask (D) 
and transferring them to the volumetric flask. Carefully rinse 
the device twice with 10 to 20 ml of water and add the rinsings 
to the solution contained in the volumetric flask. Dilute to the 
mark with water. 
5.3.2 Blank test 
Proceed as desc 
IO ml of water. ribed in 5.3.1, but replace the test portion bY 
5.4 Determination 
5.4.1 Transfer the decomposition liquid into a determination 
flask (4.3.2). Reduce the mercury( II) to metallic mercury by ad- 
ding 5 ml of the tin(ll) chloride solution (3.4). Immediately con- 
nect the air bubbling device and start the device which provides 
the air circulation. 
5.4.2 Measure the absorbance at 253,7 nm using the ap- 
paratus described in 4.3. 
5.4.3 Proceed in the same manner using the liquid derived 
from the blank test (5.3.2), and subtract the absorbance ob- 
tained from that of the liquid derived from the test portion. 
2 IS0 6637-1984 (E) 
5.5 Preparation of the calibration graph 
Into a series of six volumetric flasks (4.51, place 0 - 1 - 2 - 3 - 4 
and 5 ml of the dilute standard mercury(H) chloride solution 
(3.71, corresponding to 0 - 0,l - 0,2 - 0,3 - 0,4 and 0,5 pg of 
mercury. Introduce into each flask, in the following order, 
about 80 ml of water, 5 ml of sulfuric acid (3.1). Mix, leave to 
cool and make up to the mark with water. Transfer quan- 
titatively into six determination flasks (4.3.2). Proceed as 
described in 5.4.1 and 5.4.2. 
Plot a graph having, for example, the mercury contents, in 
micrograms, of the calibration solutions as abscissae and the 
corresponding values of absorbance as ordinates. 
5.6 Number of determinations 
Carry out two determinations on the same test sample (5.1). 
6 Expression of results 
6.1 Method of calculation and formulae 
6.1.1 Test portions taken by volume 
The mercury conten t, exp ressed 
duct as received, is equal to 
1 000 m in micrograms per litre of pro- 6.1.2 Test portions taken by mass 
The mercury content, expressed in micrograms per kilogram of 
product as received, is equal to 
1 000 m 
m0 
where 
m is the mass, in micrograms, of mercury 
tion, read from the calibration graph (5.5); in the test por- 
m0 is the mass, in grams, of the test portion. 
6.1.3 Result 
Take as the result the arithmetic mean of the values obtained in 
the two determinations (5.6), provided that the requirement for 
repeatability (see 6.2) is fulfilled. 
6.2 Repeatability 
The difference between the values obtained in the two deter- 
minations (5.61, carried out simultaneously or in rapid succes- 
sion by the same analyst on the same test sample, shall not ex- 
ceed 10 % of the mean. 
63 . Other method of expression of results 
If it is wished to express the mercury 
modify the form ulae accordi MY. 
V 
7 Test report 
m is the mass, in micrograms, of mercury 
tion, read from the calibration graph (5.5); in the test por- 
V is the volume, in millilitres, of the test portion, i.e. 
IO ml. content on the dry 
The test report shall show the method used and the result 
obtained. It shall also mention all operating details not specified 
in this International Standard, or regarded as optional, together 
with any incidents likely to have influenced the results. 
The test report shall give all the information 
complete identification of the sample. necessary for the IS0 66374984 (E) 
Dimensions in millimetres 
n 
B 
Figure 1 - Decomposition apparatus IS0 6637-1984 (E) 
Dimensions in millimetres 
19/26 
Capacity 300 ml- t4.3.21 
b-0 ext. 8 
I -3- - c- - --- 4 
--- 
-I 
- -. 
-- 
--- 
-- 
---: 
-- -- 
.- 1 
--m 
I k- 
- - 
-- - -- 
-- -- 
-- 
3F 13 r-0 ext. 6 
Absorption cell 
i4.3.11 
Figure 2 - Atomic absorption apparatus 
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